USDA is for the USA, but also everyone else. The United States Department of Agriculture provides essential information and other services to people and organizations involved in the growing of plants – including a system of identifying climate zones and plants that are hardy enough for them.
It can be puzzling to gardeners and growers outside of the USA when we try and make sense of these, not least because the designated climate zones are developed for the North American climate which has its own quirks and variances. But why to gardeners around the world use the American system at all?
A large amount of the internet is created around a US-based audience, and this is true also for information regarding plants and gardening.
If you have ever spent any time looking up plants on the internet, you will undoubtedly find hardiness information given in terms of ‘USDA Hardiness Zones’, which can be approximated to a range of minimum temperatures that the example plant could be expected to tolerate before conking out entirely.
These zones do not only identify temperature trends, but all the elements that make up the unique climates found across the USA.
Humidity and Precipitation
This is not entirely useful however as these temperatures are only survivable under particular conditions of humidity and precipitation, which may not be identical to the plants native habitat range, despite similar temperature ranges. To give one example, there are numerous ‘hardy’ succulent plants from South Africa, such as Nananthus transvaalensis, which is said to have a USDA Hardiness Zone 6 rating, meaning that it could tolerate temperatures down to -23C, and it can.
However, in order for it to tolerate such temperatures it must be bone-dry through the winter months, receiving no overhead watering, and only minimal moisture around the root-tips – and this is not unique to South African plants, but indeed even European plants such as the incredibly hardy Houseleek (Sempervivum), require very well-drained conditions to survive and thrive.
Even plants rated as Zone 7 hardy in the western US may not survive in a ‘Zone 7’ habitat on the eastern seaboard, due to differences in precipitation patterns.
Weather Patterns
It is genuinely hard to translate a USDA Hardiness zone to a different geographical location with the same minimum temperature, because the annual weather patterns can be very different, not just in terms of the amount of precipitation, but also the type (rainfall or snow), and the timings of when it occurs during the year.
Some plants may rely on brief seasonal bursts of wet weather, followed by equally important dry periods. For this reason it is important to look at each plant individually, and to consider how well adapted it is to your climate.
The provenance of a plant is extremely important, and it is often easier to stick to what grows well in your immediate area, than to push the boundary of what is possible by planting something that is badly adapted to your weather patterns.
Weather patterns can vary immensely even within the same country – and I don’t just mean huge countries like the USA or South Africa, but even tiny countries like England, where the west side receives significantly more rainfall throughout the year, and the eastern side is much drier, especially during the summer months.
The weather in the UK as a whole is very different to other European countries at the same latitude, as the UK has a temperate ‘coastal marine’ climate with relatively mild winters, and equally mild summers that rarely get very warm.
This is a big contrast with central-European countries such as Germany or Poland, which experience similar levels of winter cold but have glorious hot summers with sporadic but reasonably generous rainfall. (Global Seed Supplier Jelitto offer a fantastic zoomable climate zone map of Europe on their website)
For this reason it is possible to successfully grow the Hardy Hibiscus hybrids in mainland Europe, but they struggle in most of the UK where despite having milder winters, have summer temperatures which are actually comparatively cool and only get seriously warm later in the season.
This shortens the available growing period for plants which need warmer temperatures to kick-start their springtime growth, making it hard to grow flowering-size specimens (outside of a greenhouse), even though they can withstand incredible cold temperatures in the winter months.
We can trace this back to the ancestry of the Hardy Hibiscus Hybrids, which is a handful of wild hibiscus species from North America, growing across a range from Louisiana and Texas up to Manitoba in Canada.
There is no doubt that these plants experience incredible cold winters in their native zone, but the summers in Canada are much warmer than you might expect, and the winters are so cold that the ground remains frozen for long periods of time in the winter months, meaning that the amount of actual winter ‘wet’ experienced by the plant is less than you might expect if you are used to the cool and wet winters of the British Isles and northern coastal Europe.
This means that the ground can stay colder for longer, causing the plants to stay dormant quite late into the growing season.
How to decide which USDA zone fits your garden
According to the USDA temperature parameters for hardiness zones, most of northern Europe is technically designated between Zones 6 to 8, with the British Isles and other coastal climates being closer to the ‘Zone 8’ range (min to -9C). In practice, however, many plants that are rated as hardy to Zone 8 will promptly die in their first British winter, even though the temperatures may be well within their preferred range.
This may partly be down to a preference for COLDER winters – where the ground freezes solid, and metabolisms just grind to a halt for months on end, instead of the relatively mild spells that are sprinkled throughout the UK winters, with many days being a balmy 12 to 15C.
Closely tied to this phenomenon is the higher levels of precipitation, which tends to ride across the Atlantic Ocean on the warmer air (which causes these mild spells), and in combination they result in unsuitable growing conditions.
If a plant is not used to wetter weather, then it may have a reduced immunity to fungal and bacterial attacks that might occur in this conditions, so it may be possible to create a drier microclimate which is suitable.
In many cases it is better to choose plants that are rated as MUCH hardier than your range, so for the UK and much of Europe it is best to choose plants rated as Zone 5-6 or hardier – but it is also worth looking at the lifestyle that the plants have. If the plants go into a hard dormancy in winter then they might be fine, but if they can be tricked into weak growth by cool (but not cold) temperatures, then there may be problems ahead.
Thinking along these lines, if they want to be dry in winter – take measures to ensure this!